Light olefins, defined herein as ethylene and propylene, are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals vinyl chloride, ethylene oxide, ethyl benzene, and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenates-to-olefin (OTO) reaction process. Specifically, in an OTO reaction process, an oxygenate contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenates-to-light olefins. When methanol is the oxygenate, the process is generally referred to as a methanol-to-olefin (MTO) reaction process. Methanol is a particularly preferred oxygenate for the synthesis of ethylene and/or propylene.
Methanol is one of the major chemical raw materials, ranking third in volume behind ammonia and ethylene. Worldwide demand for methanol as a chemical raw material continues to rise especially in view of its increasingly important role (along with dimethyl ether) as a source of olefins such as ethylene and propylene and as an alternative energy source, for example, as a motor fuel additive or in the conversion of methanol to gasoline.
Methanol (as well as dimethyl ether) can be produced via the catalytic conversion of a gaseous feedstock comprising hydrogen, carbon monoxide, and carbon dioxide. Such a gaseous mixture is commonly referred to as synthesis gas or “syngas”.
Methanol is typically produced from the catalytic reaction of syngas in a methanol synthesis reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process, methanol is produced using a copper/zinc catalyst in a water-cooled tubular methanol reactor. In methanol production, syngas undergoes three reactions, only two of which are independent. These reactions are:CO+2H2→CH3OH  (A)CO2+3H2→CH3OH+H2O  (B)H2O+COH2+CO2  (C)
As can be seen from Reactions B and C, CO2 can participate in methanol synthesis. Nevertheless, it is desirable to minimize the amount of CO2 in the syngas for several reasons. In the first place, a low CO2 content in the syngas results in a more reactive mixture for methanol synthesis provided the CO2 content is at least about 2%. Furthermore, less CO2 results in lower consumption of hydrogen and lower production of water. Lower water production is useful in applications where some relative small amounts of water can be present in the methanol product such as, for example, in connection with a methanol-to-olefins (MTO) process. Production of methanol with low water content thus eliminates the need to distill water from the syngas product methanol.
The syngas stoichiometry for methanol synthesis from syngas is generally described by the following relationship known as the “Stoichiometric Number” or SN.SN=(H2−CO2)/(CO+CO2)  (D)
The value of SN theoretically required for methanol synthesis is 2.0. However, for commercial production of methanol from syngas, it is desirable that the value for SN range from about 1.95 to 2.15. Dimethyl ether (DME) may also be produced from syngas using chemistry similar to that used for methanol synthesis.
For example, U.S. Pat. Nos. 6,444,712 and 6,486,219 both describe methods for producing olefins from methanol, by way of using natural gas to make the methanol. The methods include converting the methane component of the natural gas to synthesis gas (syngas) using a steam reformer and a partial oxidation reformer. The syngas from each reformer is combined and sent to a methanol synthesis reactor. The combined syngas stream to the methanol synthesis reactor desirably has a syngas number of from about 1.4 to 2.6. The methanol product is then used as a feed in a methanol-to-olefin production process.
Autothermal reforming (ATR) involves the addition of air or oxygen with relatively smaller proportions of steam to a hydrocarbon feedstock. Reaction of hydrocarbon with oxygen proceeds according to the following general reaction schemes:CnHm+(n/2)O2nCO+(m/2)H2  (E)CnHm+(n+m/4)O2nCO2+(m/2)H2O  (F)
When methane is the hydrocarbon undergoing oxidative reforming, these reactions become:CH4+½O2CO+2H2  (G)CH4+2O2CO2+2H2O  (H)
Autothermal reforming employs both steam reforming and oxidative reforming of the hydrocarbon feed. The exothermic oxidation of the feedstock hydrocarbons generates sufficient heat to drive the endothermic steam reforming reaction over the catalyst bed. The ATR procedure is thus run at relatively high temperatures and pressures with a relatively low steam to carbon ratio. The CO2 content of the syngas from ATR processes, however, is fairly low, as is desirable for methanol synthesis.
Another known reforming process involves primarily partial oxidation of a hydrocarbon feed with an oxygen-containing gas. Catalytic partial oxidation reforming procedures are known; for purposes of this invention, partial oxidation reforming takes place in the absence of a catalyst. Due to the absence of a catalyst, partial oxidation (POX) reforming can operate at very high temperatures with little or no steam addition to the feedstock. Higher pressures than are used in ATR operations can be employed in POX reforming. However, the syngas composition resulting from POX reforming is generally deficient in hydrogen for methanol synthesis, resulting in SN and H2:CO numbers below 2. On the other hand, the CO2 content of the resulting syngas is generally very low which is below the optimum value for methanol synthesis.
Much of the methanol made today is made under high purity specifications. Grade A and grade AA methanol are commonly produced. U.S. Pat. No. 4,592,806 discloses a process for producing the grade AA methanol. The grade AA methanol has a maximum ethanol content of 10 ppm and is produced using a distillation column.
As the production of methanol continues to increase, and the new commercial uses of methanol also continue to increase, it would be advantageous to produce variable quality methanol streams, which have particular advantages for specific end uses, and which do not have to meet the stringent requirements of Grades AA and A methanol. It would also be beneficial to provide various processes for which the methanol streams would be of particular benefit.
Additionally, in many MTO reaction processes, the largest component of the oxygenate feedstock is methanol. However, relatively small amounts of oxygenates and/or higher alcohols, such as ethanol, can also be present in the feedstock. Some prior art MTO feedstocks have been treated to reduce the amounts of oxygenates and higher alcohols, while other prior art MTO feedstocks have been augmented to increase their relative content of higher alcohols and other oxygenates, for a variety of reasons. As a result, most prior art MTO processes utilize feedstocks that have undergone multiple processing and/or treatment steps to attain a higher proportion of higher alcohols, with respect to methanol.
In addition, when mixed alcohols are used as feedstocks in OTO reaction processes, water and carbon dioxide formation or retention can cause problems with olefin formation, e.g., reduced conversion efficiency. The prior art has recognized the benefit of reduction of water content and carbon dioxide content in OTO feedstock streams, but has taught complex combinations of process steps, creating increased cost and efficiency problems.
Thus, the need exists in the art for a process for producing a mixed alcohol MTO feedstock having a decreased carbon dioxide and/or water content and an increased proportion of ethanol to methanol with a minimum amount of processing.